1. Field of the Invention
The present invention relates generally to cyanide leach solutions of the type used in heap leaching processes for the recovery of precious metal values from ores and to an improved method of using lime to raise the pH such cyanide leach solutions.
2. Description of the Prior Art
The recovery of precious metal values, such as gold and silver, through the heap-leaching of low grade ores and tailings from other recovery processes is well known. In general, the procedure comprises spraying, trickling, pouring on, or otherwise applying an aqueous alkali cyanide solution to a pile of low grade ore or tailings. The aqueous alkali cyanide solution, e.g., aqueous calcium, sodium or potassium cyanide, permeates and percolates through the pile, thereby extracting the metal values as a cyanide complex. The resultant metal cyanide complex-bearing liquor is recovered from the bottom of the pile and is typically sent to an extraction and recovery facility. For example, the metal cyanide complex may be separated by adsorption on a column of activated carbon. The metal cyanide complex-bearing carbon particles are then further treated, as by electrowinning, to separate and recover the elemental metal value. The entire leaching process can sometimes take up to several weeks to finalize. Heap leaching is a relatively low cost process, and is usually most profitable when used on low grade ores.
The process of spraying, trickling, pouring on, or otherwise applying an aqueous alkali cyanide solution to a pile of low grade ore or tailings, referred to herein as cyanidation, is most common techniques for extracting gold from low grade ore. As has been briefly mentioned, when using cyanidation, heap leach gold mining operations employ dilute alkaline cyanide solutions to dissolve and transport the precious metals from the crushed ore on the leach pad to the recovery plant. The chemical reaction is referred to as the Elsner Reaction, and has a stoichiometry of:4Au+8NaCN+O2+2H2O→4NaAu(CN)2+4NaOH
Generally speaking, the cyanide heap leaching process begins with a supply of raw crushed ore, containing the precious metal values, being fed to a selected one of a cyanide leach solution pond and leach pad. The discussion which follows will use the example of a leach pad for sake of illustration. Leach pads range in size from small to large, whereby the largest pads are several hundred yards in length and width. In order to prepare the pad site, the ground is leveled and sloped toward a collection pond. A plastic liner is laid on top of a clay layer, and a layer of crushed ore is laid down to protect the liner. Now the leach pad is ready to receive heaped ore, which may be loaded onto the pad by trucks or conveyors. Once the pad has built up to some depth, for example 10 meters, a sprinkler system is installed on top of the leach pad. Next, a weak cyanide solution, referred to as the barren solution, is sprinkled over the leach pad. As the solution percolates through the heap, it dissolves the gold. The gold-bearing solution, referred to as the pregnant solution, then travels along the liner to the collection pond.
Although cyanidation is a commonly practiced technique, several known problems sometimes occur during the cyanidation of ore to recover gold and silver. Precious metals may become locked so that cyanide solutions cannot penetrate and dissolve them appropriately, leading to long leach times. Additionally, strongly adherent films on the surface of native gold and silver form during the leaching process, inhibiting or preventing further dissolution of the metals. Also, current cyanidation techniques usually require high cyanide consumption. It is even possible for the leach solution to foul, rendering it inactive for precious metal dissolution and often causing difficulties in metal precipitation from pregnant solution.
One particular problem with cyanidation is the possibility of forming toxic gases during the precipitation of precious metal from the pregnant leach solution. If the pH of the cyanide leach solution drops below about 9.3, the leach solution begins to lose hydrogen cyanide (HCN) to the air. A strong alkaline agent, like lime or caustic, is needed to maintain a cyanide-protective pH above 9.5. In some instances, the leach solution pH and alkalinity is controlled with caustic soda (NaOH). However, it is expensive to provide caustic soda in large amounts or over a continuous period of time. Replenishment of the alkalinity of the leach solution is essential because alkaline leach solutions absorb carbon dioxide from the air, converting the hydroxide alkalinity (as caustic soda) to carbonate alkalinity (as sodium carbonate), shown through the following stoichiometries:    Carbonic acid formation: H2O+CO2,gas⇄H2CO3,aq     Dissolved sodium carbonate formation: 2NaOH+H2CO3,aq⇄Na2CO3,aq+2H2O    Overall alkalinity destruction reaction: 2NaOH+CO2⇄Na2CO3,aq+H2O
As mentioned, it is critical to avoid the release of cyanide as hydrogen cyanide due to the high toxicity of this gas. Cyanide ions may become hydrogen cyanide gas when they acquire free protons, as shown by:CN−+H+=HCN(g)
Therefore, the free proton concentration is kept low by the addition of alkali such as lime or sodium hydroxide. Currently, quicklime is usually added with the crushed ore when loading the leach pads. However, when mining and leach pad loading are completed, there is no easy way to add lime to the pads since it hydrates to a pasty mass, interfering with good distribution of leach liquors. U.S. Pat. No. 4,256,706 teaches the percolation leaching of gold or silver ores by a process comprising initial agglomeration of fines in the feed by means of a binding agent and cyanide solution, followed by aging, and subsequently, leaching to recover gold or silver values. Also, the addition of lime to cyanide solutions is taught.
U.S. Pat. No. 5,336,474 also teaches a process for the leaching of gold and silver from ores and ore concentrates through contact of the ore with an aqueous leach solution containing cyanide. In addition, lime is introduced in order to adjust the pH, keeping it in levels around 8 to 13. Several pretreatment and oxidation stages are also used in this particular precious metals recovery process. The problem of fine agglomeration is addressed in a number of the prior art references. For example, U.S. Pat. No. 5,186,915 shows an agglomerating agent and method for use in heap leaching of mineral bearing ores in which a moderate to high molecular weight anionic polymer in combination with lime is used as an agglomerating agent.
During the pH adjustment of the cyanide solution through the addition of alkali, particulate solids are sometimes formed and need to be separated from the main process. If the particulate solids are not separated, the system plumbing frequently becomes plugged. There are several teachings that include separation stages in order to remove particulate solids. For example, U.S. Pat. No. 5,676,733 teaches the use of hydrocyclones during the heap leaching process of recovering precious metal values from refractory sulfide ores in order to remove the particulate solids. This method does not include the addition of lime, however.
Despite improvements of the above type in heap leach processes using alkaline cyanide solutions, a need continues to exist for further improvements in maintaining the desired pH level of the cyanide leach solution.
A need continues to exist for an improved process for using lime to raise the pH of such cyanide leach solutions, where the process more efficiently and economically regenerates hydroxide alkalinity and raises the pH levels of the cyanide leach solutions.
A need also exists for an improved process of the above type which uses lime to raise the pH of the cyanide leach solutions, which process also removes calcium carbonate precipitate without scaling or plugging the distribution system plumbing.